Process of making rigid polyurethane foam

ABSTRACT

In a process of generating a polyurethane foam by forming a mixture including isocyanate and polyol reactants, catalyst, and blowing agent, which mixture reacts exothermically to yield a rigid polyurethane foam, there is included in the mixture expandable polymer beads, made of a thermoplastic polymer (e.g., polystyrene or a polyolefin) that has a softening point at or below the maximum temperature reached during the exothermic reaction. The beads may be added in unexpanded form, partially expanded form, or fully expanded form. The heat of the urethane-forming reaction causes the beads to sinter, creating hollow polymeric spheres in the foam, which are larger than the cells in the polyurethane matrix, and which appear to improve the foam&#39;s crush resistance.

[0001] This application is a continuation-in-part of my copendingapplication, entitled “Process of Making Lightweight, Rigid PolyurethaneFoam,” which was filed Mar. 11, 2002.

[0002] This invention concerns a method of making a rigid polyurethanefoam that is cost efficient and which yields a product that isrelatively light in weight, yet has good crush resistance.

[0003] Polyurethane resins are made by reacting polyols withpolyisocyanates. The reaction is exothermic. Cross-linking, orbranching, of the polyurethane molecules can be achieved by including inthe reaction mixture some polyol molecules and/or isocyanate moleculesthat have at least three functional groups, and by adjusting the ratioof reactants accordingly. With sufficient cross-linking, rigid,thermoset polymers are obtained.

[0004] To make rigid polyurethane foam, a mixture is made of apolyfunctional isocyanate, a polyol, a blowing agent, a catalyst, and,usually, a cell-size regulator (e.g., a surfactant). A urethane-formingreaction begins once the ingredients are combined, an exotherm forms,and the blowing agent or agents cause closed cells to form in thepolymer as the mass expands and solidifies. The exotherm typicallyreaches a peak temperature of at least about 150° F. The isocyanate andpolyol reactants include enough molecules with three or more functionalgroups that the degree of cross-linking or branching is sufficient toproduce a rigid foam.

[0005] Aromatic polyisocyanates often are used when making rigid foam.Some examples are toluene diisocyanate (TDI) and polymeric isocyanate(PMDI), which is obtained by the condensation of aniline withformaldehyde.

[0006] Polyols that can be used include polyether polyols and polyesterpolyols. Propylene oxide adducts of polyfunctional hydroxy compounds oramines are one type of polyether polyols that can be used. Mixtures ofpolyester polyols and polyether polyols sometimes are employed.

[0007] Halogenated hydrocarbons, such as hydrochlorofluorocarbons andhydrofluorocarbons, can be used as blowing agents. Lower alkanes such aspentanes and cyclopentanes can be used as well. Water can also be used,as it will react with isocyanate to generate carbon dioxide in situ.Sometimes water or carbodiimide catalysts are used to generate carbondioxide as a co-blowing agent. Often the blowing agent or agents arepreblended with the polyol, together with the catalyst and the cell-sizeregulator, which usually is a surfactant.

[0008] All of this is well known to persons of ordinary skill in the artand is described, for example, in Kirk-Othmer Encyclopedia of ChemicalTechnology, 4^(th) Ed. (1997), vol. 24, pp. 695-715, which isincorporated herein by reference.

[0009] The cost of the reactants to make rigid polyurethane foam isrelatively expensive. It has now been discovered that the cost pervolume unit of the finished foam can be lowered by including in thepolyurethane reaction mixture expandable polystyrene beads (EPS) thatare made of a polystyrene that has a softening point that is equal to orbelow the maximum temperature reached by the exotherm during theurethane-forming reaction. The heat of the urethane-forming reactioncauses the polystyrene beads to sinter, while trapped in the foamingmatrix of polyurethane, creating pockets that are generally larger thanthe cells of the polyurethane. Since the polystyrene beads are lessexpensive, on a volume basis, than the foamed polyurethane, thematerials cost of the polystyrene-containing foam is less than that ofthe same volume of straight polyurethane foam. The monetary savings canbe substantial, as the cost ratio of polyurethane foam to fully expandedpolystyrene, on a volumetric basis, is currently about 25/1 to 30/1.

[0010] A side benefit of including the polystyrene beads as a filler inthe polyurethane is that the resultant foam is less dense than unfilledpolyurethane. Yet another benefit is that the polystyrene-containingfoam, despite the presence of the relatively low-cost filler (theexpanded polystyrene), appears to have the same or better crushresistance as the unfilled polyurethane.

[0011] Expandable polystyrene beads are cellular pellets of expandablepolystyrene that often are used to form lightweight molded objects.Created in a more or less granular form, and with an expanding agent inthe cells, typically the beads are pre-foamed, or “pre-expanded,” byheating to a temperature above their softening point, which often willbe in the range of about 165-185° F., until they foam to give a looseaggregate of the desired bulk density. The pre-foamed particles, whichretain their cellular structure, may then be placed in a mold or othercavity and heated with live steam, causing them to sinter and fusetogether to form a lightweight, cellular solid whose dimensionscorrespond to those of the mold cavity. When fully expanded, the beadsoften will have a diameter that is about 2 to 4 times that of theunexpanded, or “raw,” beads.

[0012] When present in the reaction mixture that forms a rigidpolyurethane foam, the EPS beads, as mentioned above, sinter. Inaddition, however, at least a substantial portion of the beads losetheir cellular structure, creating gas-filled pockets, of various sizes,in the foam, which are lined with the polystyrene of which the cellularstructure was formed. It appears that isolated spherical beads generaterelatively spherical pockets. In some sense, it might be said that thereare thin-walled polystyrene globules dispersed throughout thepolyurethane foam, as a result of the inclusion of the EPS beads in thereaction mixture.

[0013] Moreover, it appears that these polystyrene globules are coatedwith a layer of “skinned” polyurethane, i.e., a thin continuous layer ofthe polyurethane, much as is present on the outer surface of rigidpolyurethane foam moldings. The presence of these double-walled hollowstructures seems to enhance the foam's crush resistance.

[0014] Methods of making expandable polystyrene beads are well known. Asdisclosed in U.S. Pat. Nos. 3,991,020; 4,287,258; 4,369,227; 5,110,835;5,115,066; and 5,985,943, for example, all of which are incorporatedherein by reference, EPS beads may be made by polymerizing styrene in anaqueous suspension, in the presence of one or more expanding agents thatare fed at the beginning, during, or at the end of polymerization.Alternatively, they may be made by adding an expanding agent to anaqueous suspension of finely subdivided particles of polystyrene.

[0015] The expanding agent, also called a “blowing agent,” is a gas orliquid that does not dissolve the styrene polymer and which boils belowthe softening point of the polymer. Examples of suitable blowing agentsinclude lower alkanes and halogenated lower alkanes, e.g., propane,butane, pentane, cyclopentane, hexane, cyclohexane,dichlorodifluoromethane, and trifluorochloromethane. Often the beadscontain about 3 to 15%, based on the weight of the polymer, of theblowing agent. Preferably, the blowing agent will be present at a levelof about 3 to 7%.

[0016] By “beads” we here mean small particles of any geometry, e.g.,spherical, cylindrical, or lumpy. By “polystyrene” is here meant astyrene homopolymer or copolymer containing 50 wt. % or more, preferablyat least 80 wt. %, of styrene. Examples of suitable comonomers areα-methylstyrene, ring-halogenated styrenes, ring-alkylated styrenes,acrylonitrile, esters of acrylic or methacrylic acid with alcoholshaving from 1 to 8 carbon atoms, N-vinylcarbazole, and maleic acid oranhydride. A minor amount of a copolymerized chain-branching agent maybe included in the polymer as well. Suitable such agents are compoundscontaining at least two α,β-ethylenically unsaturated groups, such asdivinyl benzene, butadiene, and butanediol diacrylate. Branching agentsare generally used in an amount of about 0.005 to 0.05 mol %, based onthe styrene.

[0017] The polystyrene in the EPS beads usually has a weight averagemolecular weight in the range of about 130,000 to about 300,000.

[0018] Expandable polystyrene beads may contain other additives toimpart specific properties either to the beads or to the expandedproducts. These include, for example, flameproofing agents, fireproofingagents, nucleating agents, decomposable organic dyes, lubricants,fillers, and anti-agglomerating additives. As disclosed in U.S. Pat. No.6,271,272, incorporated herein by reference, the beads may also includeadditives, e.g., certain petroleum waxes, that quicken the rate ofexpansion when the beads are heated to expansion temperature. Dependingon the intended effect, the additives may be homogeneously dispersed inthe beads or present as a surface coating.

[0019] EPS beads come in different unexpanded particle sizes. Generally,a bead's longest dimension (e.g., its diameter), on a weight averagebasis, will be in the range of about 0.1 to 6 mm, often about 0.4 to 3mm. It is thought that unexpanded particle sizes in the range of about0.4 to 1.6 mm are preferred for the beads used in the present invention.

[0020] Unexpanded EPS beads vary as to their expansion capability, i.e.,how large they can get when heated to expansion temperature. In part,this is a function of how much blowing agent they contain. The expansioncapability of an EPS bead can be reported in terms of the bulk densityof the loose aggregate the beads will form when they are fully expanded(“fully expanded density”). By “fully expanded” is here meant theexpansion that results from the “two pass” expansion process describedin Example 2 of U.S. Pat. No. 5,115,066. This entails the use of a TriManufacturing Model 502 expander (or equivalent), operated at an inletsteam temperature of about 211° F. and an inlet steam flow rate ofapproximately 74 pounds per hour. The first-pass throughput rate isabout 208 pounds per hour. A fluidized bed drier, blowing ambient air,is used to cool the resulting prepuff. After aging for 3 hours atambient temperature and humidity, the prepuff is run through theexpander again, under the same conditions, except operating at athroughput rate of about 217 pounds per hour.

[0021] It is thought that the use of EPS beads having a capability ofreaching a fully expanded density in the range of about 0.5 to 4.5pounds per cubic foot (pcf), e.g., about 1 to 3 pcf, is preferred in thepresent invention. Examples of some commercial EPS beads that can beused in the present invention are Types 3371, 5371, and 7371 fromHuntsman Chemical and Types BFL 322, BFL 422, BF 322, BF 422, and P 240from BASF Corporation.

[0022] The EPS beads can be unexpanded, partially expanded, or fullyexpanded before being mixed with the polyurethane-forming reactants. Ifthey are unexpanded or only partially expanded, the exotherm from thepolyurethane-forming reaction will cause them to expand as well assinter, while the polyurethane resin forms, foams, and sets.

[0023] The polystyrene beads may be added to one of the polyurethanereaction mixture components prior to mixing the components, or the beadsmay be added just after the components are mixed, e.g., while theexothermic reaction is taking place. It is believed that the apparatusesand processes disclosed in U.S. Pat. Nos. 5,152,943 and 5,547,276, eachof which is incorporated herein by reference, are effective for addingexpandable polystyrene beads to a component. It is believed that theprocess disclosed in U.S. Pat. No. 6,065,862, which is incorporatedherein by reference, is effective for adding expandable polystyrenebeads to the mixture. Of course, one of ordinary skill will appreciatethat other apparatuses and processes may be used.

[0024] The amount of expandable polystyrene beads added to thepolyurethane reaction mixture in the present process preferably isenough to lower the density of the resultant foam, but not so much as toprevent the polyurethane from being the continuous phase in the finishedfoam. By “lower the density” is meant that had the same polyol andisocyanate reactants been used, in the same amounts, but without any ofthe EPS beads, the density of the resultant polyurethane foam would havebeen greater than what resulted when the EPS beads were present. In onefree rise experiment, for example, by adding 4 wt. % of fully expandedEPS beads, the density of the rigid polyurethane foam was lowered from5.5 pcf to 4.95 pcf.

[0025] It is believed to be generally preferred that the amount of theexpandable polystyrene beads be about 5% or less, e.g., about 1 to 4%,based on the combined weight of the polyurethane reactants. Amountsgreater than 4 or 5 wt.% may be difficult to uniformly mix into thepolyurethane reactants, especially if the beads are pre-expanded to,say, at least 50 or 75% of their full expansion capability. (Byexpansion to “50% of full expansion capability” is meant, herein,expansion to a bulk density that is one-half the beads'fully expandeddensity.)

[0026] The reaction and foaming of the EPS-containing reaction mixturecan take place in an unrestricted space or in an enclosure, such as amold, that will confine the mixture of foaming and expandingingredients. When the reaction and foaming occurs in a confinedenclosure, it is contemplated that the volumetric ratio of void space toreactants in the enclosure will be such as to generate during thefoaming process a maximum pressure of at least about 20 or 30 psi, andranging up to about 70 psi.

[0027] The term “polyurethane system” can be used to refer to aparticular combination of isocyanate, polyol, catalyst, blowing agent,and cell size regulator. A characteristic that helps identify anddistinguish polyurethane systems is the density of the foam a particularsystem will create when the components are mixed in an open vessel (the“free rise density”). It is thought that polyurethane systems having afree rise density of about 3 or 4 pcf to 20 or 25 pcf are generallypreferred for use in the present invention.

[0028] Examples of some commercial isocyanate/polyol pairings that canbe employed in forming polyurethane systems for use in the presentinvention are the following: Isocyanate Polyol Rated Free Rise ComponentComponent Density (pcf) Rubinate M Rimline WL 87380 8-9 Rubinate MRimline WL 87381 15-18 Baydur 645 B Baydur 645 A 5 Baydur 730 B (U 731B) Baydur 649 A 9

[0029] In the above table, the Rubinate and Rimline reactants areavailable from Huntsman Chemicals, and the Baydur reactants areavailable from Bayer Corporation.

[0030] It is thought to be generally preferred, when practicing theprocess of the present invention, that polyurethane reactants be usedthat will generate an exotherm having a peak temperature in the range ofabout 185-285° F.

[0031] The invention perhaps will be better understood by consideringthe following examples of how it can be practiced.

EXAMPLE 1

[0032] A homogeneous mixture is formed in an open vessel of 100 parts(by weight) of Rimline WL 87380 polyol, obtained from HuntsmanChemicals; 160 parts of Rubinate M isocyanate, also obtained fromHuntsman; and 10 parts of Huntsman Type 5371 expandable polystyrenebeads that have been substantially fully expanded. The polyol andisocyanate components are combined first, then the EPS beads are added.

[0033] Rimline WL 87380 is a polyether polyol formulation containingsome water. Rubinate M is an aromatic polyisocyanate. The systemgenerates CO₂ in situ as a blowing agent, through the reaction of theisocyanate with the water.

[0034] Huntsman Type 5371 EPS beads have unexpanded particle sizes thatgenerally fall in the range of 1.1 to 1.6 mm. The beads containn-pentane as the blowing agent, and they have a fully expanded densityin the range of about 1.5-3 pcf.

[0035] Once all the ingredients are mixed, the isocyanate and polyolbegin to react, the exotherm builds, and a rigid, free-rise foam isobtained. The heat of the urethane-forming reaction causes the EPS beadsto sinter. Many, if not most, of the beads create generally sphericalbubbles in the foam, ranging widely in size. Most of the bubbles havediameters in the range of about 2 to 4 mm.

[0036] If one cuts cleanly through the foam, the cross section resemblesa lunar surface, in that it is fall of craters of various sizes, createdby the cleaving in two of the many polystyrene bubbles lying in theplane of the cut. As mentioned above, the bubbles appear to be linedwith polystyrene, and these polystyrene shells appear to be covered withpolyurethane skin that is integral with the matrix of polyurethane foam.

[0037] The foam has a lower density than the rated free rise density ofthe polyurethane system used, yet the foam has excellent crushresistance.

EXAMPLE 2

[0038] The procedure of Example 1 is repeated, except that Rimline WL87381 is used as the polyol component. Once again, a rigid foam isobtained that is lower in density than the rated free rise density ofthe polyurethane system used, but which has excellent crush resistance.

[0039] It is contemplated that the present process can be performedusing any expandable polymer beads that are made of a thermoplasticpolymer that has a softening point at or below the maximum temperaturereached during the exothermic reaction. Thus, besides expandablepolystyrene beads, it is contemplated that the process can be performedusing expandable polyolefin (EPO) beads. Again, the urethane reactionmixture should be one that produces an exotherm with a temperature atleast as high as the softening point of the thermoplastic polymer ofwhich the beads are made.

[0040] As examples of expandable polyolefin beads may be mentionedexpandable polyethylene (EPE), expandable polypropylene (EPP),expandable polybutylene (EPB), and copolymers of ethylene, propylene,butylene, 1,3-butadiene, and other olefin monomers, particularlyalpha-olefin monomers having from 5 to 18 carbon atoms, and/orcycloalkylene monomers such as cyclohexane, cyclopentene,cyclohexadiene, and norbornene. Propylene/ethylene copolymers andpropylene/butylene copolymers may be preferred.

[0041] Methods of making expandable polyolefin beads are disclosed, forexample, in U.S. Pat. Nos. 6,020,388; 5,496,864; 5,468,781; 5,459,169;5,071,883; 4,769,393; and 4,675,939, all of which are incorporatedherein by reference.

[0042] Again, it is contemplated that the expandable polymer beads maybe incorporated into the urethane reaction mixture either in unexpandedform, partially expanded form, or substantially fully expanded form. Ifused in partially expanded form, they may, for example, be pre-expandedto about 50% or more of their full expansion capability, e.g., about 75%or more of full expansion size, measured as bulk density.

I claim:
 1. In a process of generating a polyurethane foam by forming amixture comprising isocyanate and polyol reactants, catalyst, andblowing agent, which mixture reacts exothermically to yield a rigidpolyurethane foam, the IMPROVEMENT wherein there is included in themixture expandable polymer beads made of a thermoplastic polymer thathas a softening point at or below the maximum temperature reached duringthe exothermic reaction.
 2. The process of claim 1, wherein the beadsare expandable polystyrene or polyolefin beads.
 3. The process of claim1, wherein the beads are expandable polystyrene beads.
 4. The process ofclaim 3, wherein the beads are substantially fully expanded prior tobeing included in the mixture.